The use of antioxidants to “stabilize” polyethylene is well known. A typical antioxidant formulation contains a “primary” antioxidant (such as a hindered phenol) which is designed to protect the polyethylene against free radical attack and a “secondary” antioxidant (such as a phosphonite; a monophosphite or a diphosphite) which is designed to quench hydroperoxides. For many applications, a simple mono-phosphite provides adequate performance as the secondary antioxidant.
Diphosphites may be used in more demanding applications—particularly where the polyethylene is exposed to a prolonged period of high temperature during the processing/conversion operations that are used to convert the polyethylene into a finished product.
Diphosphite antioxidants—as the name implies—contain two phosphorus atoms. It is generally known that these molecules are not completely compatible with linear polyethylene. In particular, the diphosphite is observed to migrate through the finished polyethylene product and form a thin coating on the surface of the product. This surface coating is highly undesirable for some products—especially films that are further processed in subsequent operations. For example, some polyethylene film is stretched when used to form a package (e.g. stretch wrap or pallet wrap) and the surface layer of diphosphite can impair the performance of the machine that stretches the film.